1. The Field of the Invention
The present invention relates generally to information displays that utilize color separation prisms. More particularly, the invention relates to a color separation prism assembly and a method for assembling the prism.
2. Present State of the Art
An effective way to achieve precise color separation for many optical applications is by use of dichroic optical interference filters, which are designed to have a sharp transition between transmission and reflection in a precise region of the visible spectrum. Dichroic filters have been used with various types of prisms, along with other optical components. When dichroic filters are utilized on the faces of prisms or prism assemblies, light corresponding to discrete spectral ranges can be re-directed or recombined.
The combination of dichroic filters and prisms is commonly used in color imaging and display systems as a way to separate colors or combine the primary colors into the final image. A compact optical element which accomplishes this purpose is known as a Philips prism. The Philips prism assembly is commonly known, and various uses thereof are described in U.S. Pat. Nos. 2,392,978, 3,659,918, 4,009,941, 4,084,180, and 4,913,528, the disclosures of which are incorporated herein by reference.
Briefly, a Philips prism assembly comprises two triangular prisms and one rectangular prism cemented into an assembly. The two triangular prism elements have an air gap between them. The rectangular prism element is optically cemented on a face thereof to a face of one of the triangular prisms, opposite the air gap bonded face. Dichroic filter coatings are on the faces of the two triangular prisms. When used in an information display system, each prism element has an associated image modulating device aligned with an external facet of each prism element. The image modulating device is usually a cathode ray tube or liquid crystal light valve. The source of light used to form the image can come from the image modulating device, as in the case of a cathode ray tube. Liquid crystal light valves (LCLV) or liquid crystal display (LCD) cells can be used with a Philips prism in a transmission mode using back lit illumination through the LCD cell into the prism.
The LCD cells are more efficient in the reflection mode, as used, for example, in image projection systems disclosed in U.S. Pat. Nos. 5,621,486 and 5,644,432, the disclosures of which are incorporated herein by reference. In this mode, a single illumination source provides white light. A polarizing beam splitter is used to direct one polarization of white light into the facet of the first prism element in the assembly. The prism splits the light into three color channels, typically red, blue, and green, which are transmitted through an exit facet of each prism element to the associated LCD. Each color channel is retroreflected back into the prism by the LCD whereby the polarization of the reflected light is spatially modulated via activation of each particular pixel comprising the LCD image plane. The light reflected from the three LCDs is recombined within the prism assembly and exits the prism through the entrance facet. A color image is formed when the retroreflected spatially modulated light enters the polarizing beam splitter, whereby light corresponding to image pixels that caused a 90 degree change in polarization is now transmitted through the polarizing beam splitter, and separated from the unmodulated light. The polarizing beam splitter acts as both a polarizer for light entering the Philips prism and an analyzer for the light exiting the prism to form a spatially modulated image. This image is then projected by an additional lens assembly onto a viewing screen.
Philips prisms are relatively compact optical assemblies for color separation. The angle of incidence for light reflected off the various dichroic coatings is less than 30 degrees, which is not the case for most other color separating prism assemblies. This can be significant, as the performance of dichroic coatings becomes increasingly angle sensitive as the angle of incidence increases.
Prism components and optical coatings are expensive to manufacture. The individual prism elements must be made within sufficiently high tolerances to obtain precise overlap of the recombined images formed at each of the liquid crystal light valves. The optical coatings are also expensive. Antireflection coatings are commonly used on the entrance face of each individual prism, and frequently on any exit face which is not cemented, or optically bonded, to another prism element. In a Philips prism, two of the three prism elements have dichroic coatings applied to their surfaces. Any errors made in prism assembly, whether they result from the assembly method or the tolerance of the individual prism elements, results in defective parts and lost value of coatings and components.